Input/output, or I/O, is circuitry that allows parts of an information processing system (such as a computer or chip) to communicate with each other. Inputs are signals received by the chip and outputs are signals sent from the chip. I/O circuitry is usually located on a periphery of the computer chip to perform as an interface between chips. Output circuitry receives signals from a core of a chip and transforms the signals electrically into a form that can be transmitted across a cable or board to another chip. Input circuitry receives signals from the cable or board and transforms signals electrically into a form that can be used by a core of the chip.
Many legacy I/O circuit designs use ECL (Emitter-Coupled Logic) circuitry. ECL is a logic family in which current is steered through bipolar transistors to compute logical functions. ECL is based on use of a multi-input differential amplifier to amplify and combine digital signals, and emitter followers to adjust DC voltage levels. As a result, none of the transistors enter saturation, nor do the transistors become completely turned off. The transistors remain entirely within their active operating regions at all times. As a result, the transistors do not have a charge storage time to contend with and can change states much more rapidly, allowing ECL circuits to operate at a very high speed. However, the transistors are continuously drawing current, which means the circuits require a lot of power.
ECL uses bipolar transistor technology rather than CMOS technology. Therefore, ECL specifications are usually written around bipolar capabilities, which are different than CMOS. For example, changes in temperature levels cause inherent shifting of input and output levels when using bipolar transistors. CMOS transistors may not be affected in the same manner. Further, CMOS integrated circuits typically operate with a power supply of 5 V or 3.3 V, while ECL traditionally operates with a negative power supply (such as −5 V). Standard ECL is not generally compatible with CMOS circuits. Accordingly, driving an ECL circuit from a CMOS circuit presents difficult design issues.
It would be desirable to replace ECL circuitry, when needed, with current CMOS circuitry and processes rather than redesigning the whole system. Thus, an I/O design or interface cell produced using CMOS processes is needed that behaves in the same manner as existing bipolar I/O circuitry.